During production of an oil-gas well, no matter whether it is a vertical well, an inclined well or a horizontal well, due to factors such as heterogeneity of oil reservoir, the oil-gas well needs to be packed off into a plurality of relatively independent zones for production. The oil-gas well production here comprises output and injection of oil-gas well fluids, such as petroleum exploitation, or injection of water, gas, chemical agents for improving a recovery rate of oil field, or the like, into the formation during production, or injection of acid liquids into the formation during some operations.
The oil-gas well is packed off into a plurality of relatively independent zones for production usually by a method of using a segmental flow-control device in combination with devices for separating the production segment of the oil-gas well into several flow units in an axial direction of the oil-gas well, for example, by a method of using a flow-control filter string plus a packer.
As we know, in the oil-gas well into which a casing is already run, an annular space is present between the casing and the well wall. If the annular space is not effectively packed off, formation fluid penetrating into the annular space will form an axial channeling flow in the annular space (those skilled in the art can all appreciate that the casing in an oil-gas well structure generally comprises a production-segment casing mainly located in a production formation, a surface casing adjacent to a well mouth and a technical casing therebetween. These kinds of casing are generally collectively called casing by those skilled in the art, and usually will not be particularly distinguished upon description because those skilled in the art all clearly understand that which segment of casing, which two segments of casing, all segments of casing or which one corresponding portion thereof the term “casing” used in a textual context specifically refers to.) In order to avoid the axial channeling flow of the formation fluid in the annular space between the casing and the well wall, currently cement is injected in to seal the annular space. This operation is briefly called well cementation.
A main purpose of well cementing operation is to prevent axial channeling flow of formation fluid in the annular space outside the casing during production.
There are many causes which may lead to undesirable quality of well cementing in the oil-gas well so that channels through which fluid can flow are present outside the casing. For example, as far as a horizontal well is concerned, one important reason for undesirable quality of well cementing is that cement slurry sinks during well cementation so that a vacancy appears in an upper portion of a cement sheath, thereby forming channeling flow passages. Existence of the channeling flow passages seriously affects the cement pack-off effect. Particularly, in the present invention, the vacancy which is outside the casing and may cause channeling is called a channeling path, which includes but not limited to one or more of vacancies not yet filled with cement outside the casing, vacancies formed by collapsing or sinking on the cement sheath (mainly when cement is not yet solidified), vacancies formed by deformation of the casing or cement sheath due to factors such as earth stress, and other vacancies which are between the casing and the well wall and may cause channeling.
FIG. 1 shows an oil-gas structure with a channeling path existing outside the casing, comprising a well wall 1, a casing 2, a cement sheath 3 provided between the casing and the well wall, a hold-down packer 4 for hanging the casing, a channeling path 5, and a plurality of perforated tunnels 6. As shown in FIG. 1, if there exists formation water-out at the perforated tunnel 6-1, water will flow into the perforated tunnel 6-1 in a direction indicated by the arrow. After passing through part of the perforated tunnel 6-1, water enters the channeling path 5, then flows in the channeling path in a direction indicated by the arrow to the perforated tunnel 6-2, flows into the casing 2 through the perforated tunnel 6-2 and thereby ruins the pack-off effect of the cement sheath.
As shown in FIG. 2, flow control is implemented by a method of running a flow-control filter string 7 into the casing by a run-in string, a hold-down packer 9 for hanging the flow-control filter string is provided at an upper portion of the flow-control filter string (e.g., those skilled in the art can appreciate that the “upper portion” of the flow-control filter string in the text refers to an end of the flow-control filter string adjacent to the borehole mouth), flow-control filters 8 are provided on the flow-control filter string, and then packers 10 are used to segment and pack off the annular space between the flow-control filter string and the casing. Due to existence of the perforations and channeling path, as shown in FIG. 2, if water appears at the perforated tunnel 6-1, the formation water, after passing through the perforated tunnel 6-1, enters the channeling path 5 and forms an axial flow in the channeling path, the water flows to the perforated tunnel 6-2, flows into the casing 2 through the perforated tunnel 6-2, the water comes to a flow-control filter 8-1 and a flow-control filter 8-2 in the casing and enters the casing through the flow-control filter 8-1 and the flow-control filter 8-2, and thereby ruins the pack-off effect of the packers 10.
Therefore, the segmental flow-control method as substantially used currently and implemented by the packers plus the flow-control filter string is not adapted for oil-gas wells with a channeling path existing outside the casing.